This invention relates generally to a heat exchange apparatus for use with a boiling liquid and a method of an apparatus for forming the enhanced surface of the heat exchanger apparatus. More particularly, this invention relates to a heat exchanger tube having a surface of integral subsurface channels having pores spaced along the surface thereof to improve the performance of such tube, and a method and apparatus wherein helical external fins forming subsurface channels are rolled over by a notched roller to form spaced pores around each helix.
Tubes manufactured in accordance with the present invention are used in a heat exchanger of the evaporator type wherein a fluid to be cooled is passed through the tubing and a boiling liquid, usually refrigerant, is in contact with the exterior of the tubing whereby heat is transferred from the fluid in the tubing to the boiling liquid. As disclosed in U.S. Pat. No. 4,425,696 an enhanced evaporator tube having subsurface channels communicating with the surroundings of the tube through openings located above an internal rib is manufactured according to a method whereby a grooved mandrel is placed inside an unformed tube and a tool arbor having a tool gang thereon is rolled over the external surface of the tube. The unformed tube is pressed against the mandrel to form at least one internal rib on the internal surface of the tube. Simultaneously, an external fin convolution is formed on the external surface of the tube by the tool arbor with the tool gang. The external fin convolution has depressed sections above the internal rib where the tube is forced into the grooves of the mandrel to form the rib. A smooth roller-disc on the tool arbor is rolled over the external surface of the tube after the external fin is formed. The smooth roller disc is designed to bend over the tip portion of the external fin to touch the adjacent fin convolution only at those sections of the external fin which are not located above an internal rib. The tip portion of the depressed sections of the external fin, which are located above the internal rib, are bent over but do not touch the adjacent convolution thereby forming a pore which provides fluid communication between the surroundings of the tube and the subsurface channels of the tube.
In U.S. Pat. No. 4,313,248 a method is disclosed for forming the heat transfer surface for a heat transfer tube whereby a finning disc forms fins on the surface of a tube and a roller disc compresses the top surface of adjacent fins downwardly to form a narrow gap between adjacent shoulders of adjacent fins.
The creation of high performance heat exchanger tubes has been pursued because it has been found that the transfer of heat to a boiling liquid is enhanced by the creation of vapor entrapment sites or cavities. It is theorized that the provision of vapor entrapment sites assist nucleate boiling. According to this theory the trapped vapor forms the nucleus of a bubble, at or slightly above the saturation temperature, and the bubble increases in volume as heat is added until surface tension is overcome and a vapor bubble breaks free from the heat transfer surface. As the vapor bubble leaves the heat transfer surface, liquid refrigerant enters the vacated volume trapping the remaining vapor and another bubble is formed. The continual bubble formation together with the convection effect of the bubbles traveling through and mixing the boundary layer of superheated liquid refrigerant, which covers the vapor entrapment sites, results in improved heat transfer.
Also, it is known that excessive influx of liquid from the surroundings can flood or deactivate a vapor entrapment site. In this regard, a heat transfer surface having a continuous gap between adjacent fins reduces the performance of the tube. Further, enhanced tubes having subsurface channels communicating with the surroundings through surface openings or pores having a specified "opening ratio", although they may prevent flooding of the subsurface channel, are generally limited to having openings for the cavities only at those locations above an internal rib or depression in the external surface of the tube.
The performance of enhanced tubes is critically dependent on the size of the subsurface channels and pores above the subsurface channels, and the number of and spacing between the pores. It is therefore important to manufacture externally enhanced tubes having consistent subsurface channels and pores around the circumference of the tube. It has been determined that in order to improve the performance of enhanced tubes the quantity of pores must be much higher than presently obtained by using an internal rib to form the pores thereabove. The present invention is generally provided with approximately eighty fores around the circumference per subsurface channel.
Thus, there is a clear need for a high performance tube having an enhanced outer surface with a plurality of subsurface channels communicating with the outside space through an increased number of evenly spaced fixed size surface pores that will, to a large extent, overcome the inadequacies that have characterized the prior art.